High voltage and high energy density rechargeable (or secondary) lithium batteries based on non-aqueous electrolytes are widely utilized in portable devices such as camcorders, notebook computers, and cell phones. This type of battery generally employs as a cathode, lithiated transition metal oxides such as LiCoO2, LiNiO2, LiMn2O4, and variations of previous oxides with dopants and a varying stoichiometry. Lithium metal, lithium alloys, and carbonaceous materials are candidates for use as anode materials for rechargeable lithium (ion) batteries. Note that as utilized herein, the term “Li batteries” generally can refer to batteries, which utilize a pure Li metal or alloy as an anode. Carbonaceous materials can be chosen over lithium metal and alloys as anode materials in commercial rechargeable batteries. This type of battery is referred to generally as “lithium battery” or “Li-ion battery” because pure lithium is not present in the anode The Li ions can be intercalated into and de-intercalated out of carbon materials during the charging and discharging processes, respectively. The advantageous of carbonaceous anodes is that they do not possess problems associated with lithium dendrite growth on the anode, which often causes shorting of the cell.
Non-aqueous electrolytes utilized in state-of-the-art lithium-ion batteries can be formed in the context of a mixed solvent system that generally includes cyclic ester compounds, such as, for example, ethylene carbonate (EC), propylene carbonate (PC), butylenes carbonate (BC), and γ-butyrolactone (i.e., gamma-butyrolactone), chain esters such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl formate, methyl formate, ethyl acetate, methyl acetate, ethyl butyrate, and methyl butyrate. Such a solvent system can contain more than one cycle ester and more than one chain ester. Cyclic esters are chemically and physically stable and possess a high dielectric constant, which is generally required for their ability to dissolve salts. The chain esters are also chemically and physically stable and possess a low dielectric constant and low viscosity, which is needed to increase the mobility of lithium ions in the electrolytes. The electrolyte solute utilized can be a lithium salt, such as, for example, lithium hexafluorophosphate (LiPF6), lithium imide (LiN(SO2CF3)2), lithium trifluoromethanesulfonate (LiCF3SO3), lithium hexafluoroarsenate (LiAsF6), lithium tetrafluoroborate (LiBF4), and lithium bis(oxalato) borate (LiBOB). The preferred salt for the state-of-the-art Li-ion batteries is LiPF6.
To improve low temperature performance, a ternary or quaternary solvent mixture containing cyclic carbonate with a high linear carbonate content can be utilized, as reported by Smart et al and Plichta et al. Refer to Smart et al., “Improved Low Temperature Performance of Lithium Ion Cells with Low Ethylene Carbonate (EC) Content Electrolytes,” Meeting Abstract, 20001 Joint International Meeting in Electrochemistry, Vol. 2001-2, San Francisco, Calif., 2-7 Sep. 2001, which is incorporated herein by reference. Additionally, refer to Plichta et al., “Low Temperature Electrolyte for Lithium and Lithium Ion Batteries,” Proc. 38th Power Sources Conference, pg 444, Cherry Hill, N.J., 8-11, June 1998, which is also incorporated herein by reference This approach is based on a low melting point and a low viscosity of the linear carbonates. Utilizing this same approach, solvents of low viscosity such as methyl acetate and methyl butyrate can also be added to improve low temperature performance.
Additionally, aliphatic carboxylate, such as methyl propionate and ethyl propionate can also be utilized to improve a low temperature performance For example, U.S. Pat. No. 5,474,862, “Nonaqueous Electrolyte Secondary Batteries,” which issued to Okuno et al on Dec. 12, 1995, and U.S. Pat. No. 5,525,443, “Non-Aqueous Secondary Electrochemical Battery”, which issued to Okuno et al on Jun. 11, 1996, indicate that methyl propionate and ethyl propionate can also be utilized to improve a low temperature performance. U.S. Pat. No. 5,474,862 and U.S. Pat. No. 5,525,443 are incorporated herein by reference. Disadvantages of such solvents include their high vapor pressure at elevated temperatures and their low dielectric constant, which restricts the ability of the solvents to dissociate salt. Li and Li-ion cells with such electrolytes typically will encounter large internal pressure when operated at elevated temperatures and under high rate discharge conditions. The lower temperature performance is not necessarily optimized utilizing electrolytes of this nature because the lithium transport number is likely to be low.
Batteries with solid-state electrolytes including polymer electrolytes are safer because they experience less pressure build-up through solvent vapor at elevated temperatures. Their poor conductivities, however, limit their application to a high temperature end. To improve solid-state polymer electrolytes for higher performance, a gel electrolyte, or a homogeneous hybrid film of polymer, salt, and plasticizers, which function as a low molecular weight polar solvent, have been utilized. For example, such a configuration is described by F. M Gray, “Polymer Electrolyte Reviews—1,” pp. 141-149, Elsevier Applied Science Publishers Ltd., New York, N.Y. 1987, which is incorporated herein by reference. Polar plasticizers include propylene carbonate (PC), ethylene carbonate (EC), γ-butyrolactone (γBL), 1-methyl-2pyrrolidinone (NMP), and dimethylsulfoxide (DMSO).
To further improve the conductivity of the gel electrolyte, solvents with low dielectric constants and low viscosities, such as diethyl carbonate (DEC) have been utilized as a major component (e.g., over 70%). Such a solvent is described in, for example, U.S. Pat. No. 5,908,717, “Electrolyte Composition for Rechargeable Electrochemical Cells,” which issued to Pendalwar et al on Jun. 1, 1999. U.S. Pat. No. 5,908,717 is incorporated herein by reference. U.S. Pat. No. 5,908,717 discloses an electrochemical cell that includes first and second electrodes and an electrolyte system disposed therebetween. The electrolyte system includes a polymeric support structure through which is dispersed an electrolyte active species in an organic solvent. The solvent, which remains liquid to low temperatures, is a binary or higher order system comprising diethyl carbonate and one or more of propylene carbonate, ethylene carbonate, dimethyl carbonate, dipropylcarbonate, dimethylsulfoxide, acetonitrile, dimethoxyethane, tetrahydrofuran, n-methyl-2-pyrrolidone, and combinations thereof.
Among the lactam compounds, NMP can also be utilized as a complexing agent or a plasticising agent for fabricating solid-state polymeric and gel polymeric electrolytes utilizing a specific salt, such as alkali metal triflate (or trifluoromethanesulfonate) salt. Such a use of NMP is described in U.S. Pat. No. 6,025,096, “Solid State Polymeric Electrolyte for Electrochemical Devices,” which issued to Stephen F. Hope on Feb. 15, 2000, and is incorporated herein by reference. U.S. Pat. No. 6,025,096 discloses a solid state polymeric electrolyte, which is formed by complexing an alkaline metal triflate salt and polyethylene oxide with an ester and an ether or a pyrrolidinone and an ether, or two ethers of different boiling points as co-solvents to form a solid or semi-solid state electrolyte.
NMP is commonly utilized as a solvent for dissolving polyvinylidene fluoride binder. Mixing the binder solution with active electrode material, such as lithiated transition metal oxides for positive electrode or carbonaceous materials for negative electrode and conductive diluents, can form a slurry thereof. Such a slurry can then be coated on a metal substrate and the NMP removed later by heating under reduced pressure. Such a method is commonly utilized in lithium battery industrial processes for fabricating coated electrodes on current collectors An example of such a method is described by DA. Stevens and J. R. Dahn, “The Mechanisms of Lithium and Sodium Insertion in Carbon Materials,” J. Electrochem. Soc, 148(8), A803 (2001), which is incorporated herein by reference
NMP can also be utilized as a solvent of choice for the preparation of electro conductive polymer composite electrodes for use in secondary batteries as a positive electrode. An electro conductive polymer such as polyaniline and the polymer electrolyte, which can be composed of, for example, LiClO4 and polyvinyl alcohol or polyalkylene oxide, can be dissolved in NMP to form a solution thereof The resulting solution can then be cast on a current collector and the solvent later removed and a film electrode formed thereof. Such a technique is described in U.S. Pat. No. 5,863,454, “Electroconductive Polymer Composites for Use in Secondary Batteries as Positive Electrode Materials,” which issued on Jan. 26, 1999 to Chen et al, and which is incorporated herein by reference.
Other lactams utilized in industry include 1-methyl-piperidone and 1-methyl caprolactam, which are important monomers for synthetic fabrics. Lactams or solvents based on lactam structures are not utilized in liquid based electrolytes for Li-ion batteries.